The manufacturing process for producing elastomeric articles from natural or synthetic rubber latex involves a curing step during which cross linking or vulcanisation occurs through sulphur groups between the polymer units.
Conventional processes for making elastomeric articles from natural or synthetic latex typically involve preparing a latex precompound, dispersion or emulsion, obtained by mixing latex, sulphur, an activator and an accelerator system, maturation for a certain period, dipping a former in the shape of the article to be manufactured into a coagulant and subsequently into the latex precompound and curing the latex present on the former.
Desirable properties of certain elastomeric articles such as tensile strength are substantially affected by the cross linking and curing stages of the manufacturing process.
The use of sulphur or sulphur containing compounds as vulcanising agent and zinc oxide as activator, the use of mixtures of inorganic salts (calcium nitrate, calcium carbonate and surface active agents) as a coagulator, and the use of vulcanising or sulphur cross linking accelerator compositions in the manufacture of rubber articles, is well known.
Conventional vulcanisation accelerators include dithiocarbamates, thiazoles, guanidines, thioureas and sulphenamides.
Certain fields, in which elastomeric articles are needed, such as the medical, health care or personal hygiene field, utilized specific types of equipment and processing techniques which enables the specific performance and regulatory requirements of the particular article produced.
The use of natural rubber latex in the manufacture of certain article such as medical gloves has been associated with disadvantageous properties such as allergic reactions, generally believed to be caused by natural proteins or allergens present within the natural rubber latex and the final product.
Synthetic elastomeric products and manufacturing processes which altogether reduce or avoid the likelihood of potential adverse reactions of the user or wearer, are of increasing interest in the medical field, particularly in the field of gloves.
A majority of glove manufacturing processes are water-based dipping systems.
It is generally known that solvent-based systems can possibly be used for poly(isoprene) and other elastomers although such solvent-based systems are poorly suited for the manufacture and molding of elastomeric articles for medical applications. One difficulty in the field of gloves for example is the design of processes and materials which will produce a thin elastomeric article having desirable properties such as high tensile strength. Another disadvantage of solvent-based systems is solvent toxicity.
Process and materials that would obviate or reduce the need for the use of toxic solvents, while at the same time yielding a product having desirable properties for medical applications, are thus still being explored.
More recently a process was proposed (WO 02/090430) for the manufacture of elastomeric poly(isoprene) articles such as medical gloves, condoms, probe covers, catheters, comprising the steps of:    1. preparing an aqueous latex composition containing an accelerator composition and a stabilizer, said accelerator composition comprising a dithiocarbamate, a thiazole and a guanidine compound;    2. dipping a former into said compounded latex composition; and    3. curing said compound latex composition on said former to form said elastomeric poly(isoprene) article.
Preferably said accelerator composition comprised zinc dietnyidithiocarbamate, (ZDEC), zinc-2-mercaptobenzothiazole (ZMBT) and diphenylguanidine (DPG).
Although the use of said accelerator composition represented an improvement of the manufacturing process, it has been found that all these accelerators in the up till now applied concentrations are capable of producing Type IV allergic response in human beings and may also possess increasingly unacceptable eco-tox and acute toxicity profiles in the concentrations used until now. In addition ZDEC and ZMBT have beers found to produce potentially harmful N-nitrosamines as is known from e.g.
“Safer Accelerators for the Latex industry”, Sakroborty K. B. and Couchman R., Latex 2004, Hamburg, 20-21 Apr. 2004, p. 75-87 and in particular p. 75 and 76. From the same publications (p. 82) were also known accelerator compositions which comprised ZDEC, MBT and DPG in amounts to provide concentrations in the latex to be cured of 1.0, 1.0 and 0.5 phr of dry rubber respectively.
Another characteristic of said accelerator compositions was that actually a stabilizer had to be used in conjunction with said accelerator composition to prolong the stability or pot-life of the procured poly(isoprene) latex.
Moreover, from WO 2005/035539A2 accelerator compositions for synthetic poly(isoprene) were known which comprised a dithiocarbamate and thiourea and preferably in addition a thiazole. As preferred dithiocarbamate component was mentioned 1,3-dibutyl thiourea.
A clear teaching in said document is that preferred accelerator systems did not contain tetramethylthiuram disulfide or diphenylguanidine.
From U.S. Pat. No. 2,684,391 the use of a vulcanising accelerator, comprising a metal salt of an N-substituted dithiocarbamic acid and a small proportion of an amine, relative to the dithiocarbamate, was known. As preferred amounts of said amine were specified from 2% to 25% by weight of the dithiocarbamate in the accelerator. As a preferred dithiocarbamate zinc diethyl dithiocarbamate was used and as preferred amine component diphenylguanidine was used. Moreover, it was stated that the diethyl dithiocarbamate could be mixed with another dithiocarbamate accelerator.
From GB-1211938 was known a process for the production of natural or synthetic cellular rubber or a blend thereof, which comprised beating a mixture comprising foamed rubber latex, vulcanising agent and gelling agent (having been added to the already foamed latex), by means of a high frequency dielectric heating to a temperature below 100° C. so that, there is substantially no evolution of steam, thereby gelling the foamed rubber latex and then vulcanising the gelled rubber latex foam thus obtained in the absence of high frequency dielectric heating.
An accelerator system which comprised zinc diethyl dithiocarbamate, diphenylguanidine and zinc-2-mercapto benzothiazole, was used, wherein the specified weight ratio of phenylguanidine relative to the weight of unsaturation. Said accelerator system comprised at least two different dithiocarbamates, one of the components being zinc diethyl dithiocarbamate, a thiazole and a guanidine such as diphenylguanidine. No reference was actually made to coagulant dipping.
From WO 03/072340 an poly(isoprene) article was known which comprised a poly(isoprene) latex, a curing compound comprising sulpher, a thiuram compound and a second compound, which inter alia could be xanthogen sulfide plus zinc dibenzyl dithiocarbamate or diisopropyl xanthogen plus zinc dibenzyl dithiocarbamate. A preferred second compound component was zinc dibenzyl dithiocarbamate. Moreover, said document actually taught in par. [0019] that the surface of condoms prepared from poly(isoprene) formulations, including ZDEC and ZBDC, was inferior.
It will be appreciated that there is still a strong need for an improved manufacturing process for gloves etc. from synthetic rubber latex, providing films from a stable precompound of latex, showing the required mechanical properties (tensile strength ≧17 MPa; elongation at break ≧650%) within a cycle time of 1 day, by using less amounts of vulcanising agent and in particular sulphur, a safe accelerator system and a maturation time from some hours to less than 2 days.
As result of extensive research and experimentation said improved manufacturing process and acceleration system to be used therein, have surprisingly been found.